WASHINGTON, Oct. 26 — The divisive debate over gay marriage, which played a prominent role in 2004 campaigns but this year largely faded from view, erupted anew on Thursday as President Bush and Republicans across the country tried to use a court ruling in New Jersey to rally dispirited conservatives to the polls.

Wednesday’s ruling, in which the New Jersey Supreme Court decided that gay couples are entitled to the same legal rights and financial benefits as heterosexual couples, had immediate ripple effects, especially in Senate races in some of the eight states where voters are considering constitutional amendments to ban gay marriage.

President Bush put a spotlight on the issue while campaigning in Iowa, which does not have a proposal on the ballot. With the Republican House candidate, Jeff Lamberti, by his side, Mr. Bush — who has not been talking about gay marriage in recent weeks — took pains to insert a reference into his stump speech warning that Democrats would raise taxes and make America less safe.

“Yesterday in New Jersey, we had another activist court issue a ruling that raises doubts about the institution of marriage,” Mr. Bush said at a luncheon at the Iowa State Fairgrounds that raised $400,000 for Mr. Lamberti.

The president drew applause when he reiterated his long-held stance that marriage was “a union between a man and a woman,” adding, “I believe it’s a sacred institution that is critical to the health of our society and the well-being of families, and it must be defended.”

The ruling in New Jersey left it to the Legislature to decide whether to legalize gay marriage. Even so, the threat that gay marriage could become legal energized conservatives at a time when Republican strategists say that turning out the base could make the difference between winning and losing on Nov. 7. With many independent analysts predicting Republicans will lose the House and possibly the Senate, President Bush’s political team is counting on the party’s sophisticated voter turnout machinery to hold Democratic advances enough that Republicans can at least maintain control.

“It’s a game of margins,” said Charles Black, a Republican strategist who consults frequently with Karl Rove, the chief White House political strategist. “You’ve got about 20 House races and probably half a dozen Senate races that are either dead even or very, very close. So if it motivates voters in one or two to go vote, it could make a difference.”

Democrats predicted Thursday that the debate would not dramatically alter the national conversation in an election that has been dominated by the war in Iraq and corruption and scandal in Washington. But across the country, Republicans quickly embraced the New Jersey ruling as a reason for voters to send them to Capitol Hill.

In Virginia, the court decision could not have come at a better time for Senator George Allen, a Republican whose campaign for re-election had been thrown off course by allegations that he had used racially insensitive remarks. The Virginia ballot includes a proposed constitutional amendment to ban gay marriage. Mr. Allen supports it; his Democratic opponent, Jim Webb, argues that the ban is unnecessary.

On Thursday, Mr. Allen could be found in Roanoke at a rally held by backers of a ballot initiative to ban gay marriage. Victoria Cobb, an organizer of the events, said the New Jersey ruling was giving the cause “a new momentum.”

“It’s an issue that’s going to play a big role in the next 12 days,” Mr. Allen’s campaign manager, Dick Wadhams, said in an interview.

In Tennessee, another state with a proposal to ban gay marriage, Representative Harold E. Ford Jr., a Democrat running for the Senate, was sparring with Republicans over an advertisement in which the Republican National Committee asserts that Mr. Ford supports gay marriage — an assertion Mr. Ford says is wrong. On Thursday, he responded with his own advertisement, calling the Republican ad “despicable, rotten lies.”

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Mr. Ford says he will vote for the Tennessee gay marriage ban. With early voting under way, the Republican candidate, Bob Corker, is telling voters that he has already cast his ballot in favor of the gay marriage ban.

And in Pennsylvania, where Senator Rick Santorum, the Senate’s leading Republican backer of a gay marriage ban, is fighting for his political survival, conservative advocacy groups were working furiously to revive the gay marriage debate. Pennsylvania does not have a ballot initiative.

“It’s an important wedge issue to talk about between candidates where there are two distinct viewpoints on the issue,” said Joseph Cella, president of Fidelis, a national Catholic advocacy group that has embraced Mr. Santorum for his views on abortion and gay marriage. Mr. Cella said his organization, which was also working to pass a gay marriage ban in Colorado, was contemplating an advertising campaign.

As of January 2006, 45 states had enacted some form of law — from a simple statute to a constitutional amendment — banning same-sex marriage. In addition to Virginia, Tennessee and Colorado, the states that have proposed constitutional amendments on the November ballot include Arizona, Idaho, South Carolina, South Dakota and Wisconsin.

For conservatives, the debate brings back memories of 2004, when they rallied in opposition to a Massachusetts court ruling that same sex couples had a right to marry. The issue proved central in places like South Dakota, where Senator John Thune, a Republican, railed against activist judges in his successful campaign to oust Tom Daschle, then the Senate Democratic leader.

This year, by contrast, conservatives have felt frustrated that the debate over gay marriage and the judiciary is no longer front and center.

“I think they’ve been a little sedate,” Mr. Cella said. But in the wake of the New Jersey ruling, he said, conservatives “are really getting motivated, and this is a shot in the arm to propel that.”

Democrats, though, insist they are not concerned.

“It’s not going to be close to the issue it was in 2004,” said Senator Charles E. Schumer, Democrat of New York. “In 2004 they scared people that the court ruling in Massachusetts would just change America and families dramatically. By 2006, it’s clear that hasn’t happened, and so the scare tactic, what motivated people to go to the polls, just isn’t there.”

One place the New Jersey court ruling is not likely to have much of a political impact is, paradoxically, New Jersey, a largely Democratic state that does not have a proposed gay marriage ban on the ballot.

The Republican Senate candidate, State Senator Thomas H. Kean Jr., has been distancing himself from his party throughout the campaign, in which he has focused largely on economic issues, domestic security and alleged ethical improprieties on the part of his Democratic opponent, Senator Robert Menendez. A Kean spokeswoman said Thursday that theme is unlikely to change.

“We’re going to stick with the issues that we’ve been winning on this entire campaign,” the spokeswoman, Jill Hazelbaker, said. Gay marriage, she said, “is not an issue that he’s not talking about, or that he’s trying to avoid. But in terms of our marquee issues that we’re winning on, I don’t think it rises to an issue that’s going to define the campaign.”

A version of this article appears in print on , on Page A1 of the New York edition with the headline: G.O.P. MOVES FAST TO REIGNITE ISSUE OF GAY MARRIAGE. Order Reprints|Today's Paper|Subscribe